Fracture resistance of mineral reinforced polyamide 6 Christopher J.G. Plummer a, * , Muriel Mauger b , Philippe Be ´guelin b , Gilles Orange c , Joe ¨l Varlet d a Ecole Polytechnique Fe ´de ´rale de Lausanne (EPFL), Laboratoire de Technologie des Composites et Polyme `res (LTC), Institut des Mate ´riaux, Lausanne CH-1015, Switzerland b Ecole Polytechnique Fe ´de ´rale de Lausanne (EPFL), Laboratoire des Polyme `res (LP), Institut des Mate ´riaux, Lausanne CH-1015, Switzerland c CRA, Rhodia-Recherches, 52 rue de la Haie Coq, 93308 Aubervilliers, France d CRL, Rhodia-Recherches, 85 rue des Fre `res Perret, BP 62-69192 Saint-Fons Cedex, France Received 29 July 2003; accepted 2 December 2003 Abstract Tensile tests have been carried out over a wide range of test speeds on compact tension specimens of polyamide 6 containing spherical silica particles, whose size and content had been adjusted to give optimum impact performance in conventional impact tests after conditioning at 50% relative humidity. The tensile test results confirmed there to be a significant improvement in the high speed crack initiation resistance at room temperature and at high moisture contents on addition of the silica particles. However, at low moisture contents and/or temperatures well below the glass transition temperature, the crack initiation resistance was reduced. It is hence inferred that for the chosen silica particle distribution, toughening requires a certain minimum level of matrix ductility in order to be effective. q 2003 Elsevier Ltd. All rights reserved. Keywords: Polyamide 6; Fracture; Mineral reinforcement 1. Introduction Commercial thermoplastics often contain a mineral filler, whose function is typically to reduce their price and increase their dimensional stability, stiffness, heat deflection tem- perature and, in some cases, impact resistance. Optimization involves controlling the size distribution, geometry, dis- persion and spatial separation of the filler particles and the properties of the interface between the particle and the polymer matrix. However, the relationship between these parameters and the impact resistance of mineral filled polymers remains unclear, and there are relatively few examples of polymers whose impact performance has been significantly improved by filler addition. Even in the case of rubber modified polymers, which have been extensively investigated over the last 20 years or so [1–3], current practice is as much based on empirically established criteria as it is on a detailed understanding of the underlying mechanisms. There is nevertheless a broad consensus according to which the role of the modifier particles is to activate deformation modes capable of dissipating energy in an extended damage zone around stress concentrations when the material is subject to mechanical loading. In thermoplastics, plasticity may take the form of homo- geneous shear or crazing, depending on the matrix and the stress state. The formation of crazes is associated with globally brittle behaviour in unmodified polymers because it is highly localized and leads to limited energy dissipation [4]. It is therefore sometimes argued that the role of the modifier in brittle thermoplastics is to promote the formation of large numbers of crazes in the vicinity of a stress concentration, so that there is an increase in overall energy dissipation. However, even in systems where rubber modification leads to a significant increase in the craze density during tensile deformation, the accompanying increase in toughness may include a significant contribution from homogeneous shear deformation, favoured by local constraint release in the presence of cavities and crazes [5, 6]. If localized deformation is activated by cavitation or debonding of the modifier, it is widely assumed that this should ideally occur when the local ligament stress is close to the effective s y : In rubber toughened polymers, the cavitation stress is controlled by adapting the particle size, 0032-3861/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2003.12.022 Polymer 45 (2004) 1147–1157 www.elsevier.com/locate/polymer * Corresponding author. Tel.: þ 41-21-693-28-56; fax: þ 41-21-693-58- 68. E-mail address: christopher.plummer@epfl.ch (C.J.G. Plummer).